Electrically Conductive Crown Architecture for a Tire of a Heavy Duty Civil Engineering Vehicle

ABSTRACT

A radial tire (10), with the sidewalls thereof (20), and the tread thereof (30) arranged for minimizing the temperature of the tire while guaranteeing its electrical conductivity. The tread (30) comprises two wings (311, 312) and a central portion (32). These components rest on a base layer (33) radially on the inside of the tread (30). The base layer (33) contains a lateral portion (331, 332) partly in contact with a tread wing (311, 312). This structure of the crown of the tire, in contact with the carcass reinforcement makes it possible to constitute a preferential conductive pathway of the electric charges between the rim and the ground when the tire is mounted on its rim and flattened on the ground.

The present invention relates to a radial tire intended to be fitted toa heavy vehicle of construction plant type, and more particularly to thesidewalls and to the tread of such a tire.

A radial tire for a heavy vehicle of construction plant type is intendedto be mounted on a rim, the diameter of which is at least equal to 25inches, according to European Tire and Rim Technical Organisation orETRTO standard. It is usually fitted to a heavy vehicle, intended tobear high loads and to run on harsh terrain such as stone-coveredtracks.

Generally, since a tire has a geometry of revolution relative to an axisof rotation, its geometry is described in a meridian plane containingits axis of rotation. For a given meridian plane, the radial, axial andcircumferential directions respectively denote the directionsperpendicular to the axis of rotation, parallel to the axis of rotationand perpendicular to the meridian plane.

In the following text, the expressions “radially inner/radially on theinside” and “radially outer/radially on the outside” mean “closer to”and “further away from the axis of rotation of the tire”, respectively.“Axially inner/axially on the inside” and “axially outer/axially on theoutside” mean “closer to” and “further away from the equatorial plane ofthe tire”, respectively, the equatorial plane of the tire being theplane passing through the middle of the running surface andperpendicular to the axis of rotation.

The top end of a component of the tire refers to the radially outer endof said component. Conversely, the bottom end refers to the radiallyinner end of said component.

A tire comprises a tread intended to come into contact with the ground,the two axial ends of which are connected via two sidewalls to two beadsthat provide the mechanical connection between the tire and the rim onwhich it is intended to be mounted.

A radial tire further comprises a reinforcement made up of a crownreinforcement radially on the inside of the tread and a carcassreinforcement radially on the inside of the crown reinforcement.

The crown reinforcement of a radial tire comprises a superposition ofcircumferentially extending crown layers radially on the outside of thecarcass reinforcement. Each crown layer is made up of generally metallicreinforcers that are mutually parallel and coated in a polymericmaterial of the elastomer or elastomeric compound type.

The carcass reinforcement of a radial tire customarily comprises atleast one carcass layer comprising generally metallic reinforcers thatare coated in an elastomeric compound. A carcass layer comprises a mainpart that joins the two beads together and is generally wound, in eachbead, from the inside of the tire to the outside around a usuallymetallic circumferential reinforcing element known as a bead wire so asto form a turn-up. The metallic reinforcers of a carcass layer aresubstantially parallel to one another and form an angle of between 85°and 95° with the circumferential direction.

A tire sidewall comprises at least one sidewall layer consisting of anelastomeric compound and extending axially towards the inside of thetire from an outer face of the tire, in contact with the atmosphericair. At least in the region of greater axial width of the tire, thesidewall extends axially inwardly to an axially outermost carcass layerof the carcass reinforcement.

An elastomeric compound is understood to mean an elastomeric materialobtained by blending its various constituents. An elastomeric compoundconventionally comprises an elastomeric matrix comprising at least onediene elastomer of the natural or synthetic rubber type, at least onereinforcing filler of the carbon black type and/or of the silica type, ausually sulfur-based crosslinking system, and protective agents.

An elastomeric compound may be characterized mechanically, in particularafter curing, by its dynamic properties, such as a dynamic shear modulusG*=(G′²+G″²)^(1/2), wherein G′ is the elastic shear modulus and G″ isthe viscous shear modulus, and a dynamic loss tgδ=G″/G′. The dynamicshear modulus G* and the dynamic loss tgδ are measured on a viscosityanalyser of the Metravib VA4000 type according to standard ASTM D5992-96. The response of a sample of vulcanized elastomeric compound inthe form of a cylindrical test specimen with a thickness of 4 mm and across section of 400 mm², subjected to a simple alternating sinusoidalshear stress, at a frequency of 10 Hz, with a deformation amplitudesweep from 0.1% to 50% (outward cycle) and then from 50% to 0.1% (returncycle), at a given temperature, for example equal to 60° C., isrecorded. These dynamic properties are thus measured for a frequencyequal to 10 Hz, a deformation equal to 50% of the peak-to-peakdeformation amplitude, and a temperature that may be equal to 60° C. or100° C.

An elastomeric compound may also be characterized by its electricalresistivity which characterizes the ability of the compound to let theelectrical charges move freely, and therefore to allow the flow of anelectrical current. The electrical resistivity is generally denoted byp, and its unit of measurement is in Ohm·metre (Ω·m) but it is common,in the field of tires, to express the measurement of the electricalresistivity in Ohm·centimetre (′Ω·cm). The test for measurement of theelectrical resistivity is described, for example, in the standardASTM-D257. An electrical resistivity of 1 Ω·m, or of 10² ′Ω·cm,corresponds to the resistance to the flow of the electric current in acylindrical portion of compound having a length of 1 m and a crosssection of 1 m². The electrical conductivity is the inverse of theelectrical resistivity, denoted by σ and satisfying σ=1/ρ. Subsequently,use will be made of either the electrical conductivity σ or theelectrical resistivity ρ, depending on the context, to characterize theelectrical properties of the compounds.

A material that is very weakly electrically conductive or that iselectrically resistant is understood to mean a material having anelectrical resistivity of greater than 10⁸ ′Ω·cm. Similarly, anelectrically conductive material is understood to mean a material havinga resistivity of less than 10⁶′Ω·cm. These materials may or may not beelastomeric compounds.

The electrical resistivity properties of the elastomeric compounds aredirectly linked to their composition and in particular to the use ofreinforcing fillers. It is known that an amount of from 35 to 45 phr(parts per hundred parts of elastomer) of carbon black is sufficient togive an elastomeric compound a resistivity sufficient to dischargeelectrostatic charges.

It is also known that a combination of reinforcing fillers of carbonblack type and of silica type, in suitable proportions, favours thereaching of a performance compromise between the rolling resistance andthe endurance of the tire, by lowering the temperature level. However,if the amount of carbon black is less than 35 phr, the elastomericcompound is electrically insulating.

By way of illustration, a tread elastomeric compound with a reinforcingfiller comprising at least 40 phr of silica, and at most 10 phr ofcarbon black has an electrical resistivity of the order of 10¹²′Ω·cm.

Furthermore, the thermal conductivity or conductibility of a material isa physical quantity that characterizes the ability of the material toallow heat transfer by conduction. It represents the amount of heattransferred per unit of area and of time, under a temperature gradientof 1 degree Kelvin or 1 degree Celsius and per metre. In theInternational System of Units, the thermal conductivity is expressed inwatt per metre·Kelvin (W·m⁻¹·K⁻¹).

Thus, a thermal conductivity of 1 W·m⁻¹·K⁻¹ represents the amount ofheat which propagates through a material by thermal conduction, acrossan area of 1 m², over a distance of 1 m. The measurement of the thermalconductivity on a test specimen of elastomeric compound is described,for example, in the standard ASTM-F433.

Just like for the electrical conductivity, the thermal conductivity isdirectly linked to the composition of the elastomeric compounds. Theheat transfer by conduction is carried out by virtue of the reinforcingfillers. Thus, by way of illustration, a tread elastomeric compoundcomprising a reinforcing filler comprising at least 40 phr of silica andat most 10 phr of carbon black is an electrically insulating compound,that is endowed with a low thermal conductivity.

With the aim of improving the rolling resistance and therefore ofreducing the fuel consumption, the tires on the market often compriseelastomeric compounds predominately comprisingnon-electrically-conductive reinforcing fillers such as silica, or elseelastomeric compounds weakly loaded with electrically-conductivereinforcing filler such as carbon black.

The use of these elastomeric compounds has thus been widely developedfor the creation of treads given the advantages afforded by suchcompounds in also improving the performance relating to the grip on dry,wet or icy ground, the wear resistance, or else the running noise. Thistype of tire is described by way of illustration in European patentapplication EP-501 227.

However, the use of these elastomeric compounds is accompanied by adifficulty linked to the accumulation of static electricity when thevehicle is running, and to the absence of flow of these charges to theground due to the very high resistivity of the elastomeric compoundsconstituting said tread. The static electricity thus accumulated in atire is capable of causing, when certain particular conditions are met,an electric shock to the occupant of a vehicle, when he or she has totouch the body of the vehicle. This static electricity is, moreover,capable of accelerating the aging of the tire due to the ozone generatedby the electrical discharge. It may also be the cause, depending on thenature of the ground and the vehicle, of a poor operation of thebuilt-in radio in the vehicle due to the interferences that itgenerates.

This is the reason why many technical solutions have been proposed toenable the flow of the electrical charges between the crown of the tireand the ground.

However, these known technical solutions usually consist in connectingthe tread to a portion of the tire, such as the sidewall, a crownreinforcement layer or a carcass reinforcement layer, which haselectrically-conductive properties. The electric charges are thereforedischarged to the ground from the rim, connected to the vehicle, bysuccessively passing through the bead of the tire in contact with therim, the sidewalls and more particularly the elastomeric coatingcompounds of the carcass layer reinforcers or at least one sidewallelastomeric compound, and finally the crown reinforcement and the tread.

The thermomechanical study of a tire for a construction plant vehicleshows that the viscoelastic losses of the elastomeric compounds aresources of heat, the intensity of which depends on the volume of theelastomeric compounds and on the deformations that they undergo. Thisheat, which is generated when the tire is in motion, is discharged intothe environment more or less quickly depending on the thermalconductivity values of each material of the tire. When the thermalconductivity of an elastomeric compound is too low, the heat accumulatesand results in the bakelization thereof. The tire then loses its elasticproperties, which is unfavourable for the use thereof.

The optimization of the endurance of a tire for a construction plantvehicle requires maintaining the operating temperature at a suitablelevel. The control of the temperature level is directly related to theviscoelastic properties of the compounds which depend on the compositionthereof, and especially on the amount of reinforcing fillers.Ultimately, the optimization of the endurance of the tire leads to acoupled problem where the physical parameters involved are the viscousshear modulus, or the viscoelastic loss, which is connected to theviscoelastic heat sources, the thermal conductivity which controls theconduction of the heat in the elastomeric compounds, and the electricalconductivity which must be at a level sufficient for dischargingelectrostatic charges.

In a tire for a construction plant vehicle, the tread represents around35% to 40% of the total volume of rubber of the tire, and the sidewallsaround 15% of this same volume. The tread being subjected to the shearstresses of the ground is the site of large-amplitude strains. Asregards the sidewalls which are subjected to bending cycles during theuse of the tire, the shear strains are also sizeable. The inventors havetherefore focused on these two zones of high mechanical stresses inorder to determine the optimal compositions of the elastomeric compoundsto meet the desired performance compromise between minimal thermal leveland ability to discharge the electrostatic charges.

The inventors have thus set themselves the objective of improving theendurance of a tire for a construction plant vehicle, limiting itsaverage operating temperature to an appropriate level of around 90° C.,while guaranteeing its ability to be electrically conductive, i.e. todischarge the electrostatic charges.

This objective has been achieved by a tire for a heavy vehicle ofconstruction plant type, comprising:

-   -   a tread comprising two axial end portions or tread wings axially        separated by a central portion;    -   a base layer, radially on the inside of the tread, comprising at        least one lateral portion at least partly in contact with the        tread wing;    -   a crown reinforcement, radially on the inside of the base layer,        comprising at least one crown layer, consisting of metallic        reinforcers that are coated in an electrically-conductive        elastomeric compound;    -   two sidewalls connecting the tread wings to two beads, intended        to come into contact with a mounting rim by means of a bead        layer made of electrically-conductive elastomeric compound;    -   each sidewall being axially on the outside of a carcass        reinforcement comprising at least one carcass layer consisting        of metallic reinforcers that are coated in an        electrically-conductive elastomeric coating compound;    -   at least one tread wing consisting of a first elastomeric        compound M₁ having a thermal conductivity λ₁ and an electrical        resistivity ρ₁;    -   the central tread portion consisting of a second elastomeric        compound M₂ having a viscoelastic loss tgδ₂;    -   the base layer consisting of a third elastomeric compound M₃        having a thermal conductivity λ₃ and an electrical resistivity        ρ₃;    -   each sidewall consisting of a fourth elastomeric compound M₄        having a viscous dynamic shear modulus G″₄;    -   the first elastomeric compound M1 of at least one tread wing has        a thermal conductivity λ₁ at least equal to 0.190 W/m·K;    -   the second elastomeric compound M₂ of the central tread portion        has a viscoelastic loss tgδ₂ at most equal to 0.06;    -   the third elastomeric compound M₃ of the base layer has a        thermal conductivity λ₃ less equal to 0.190 W/m·K;    -   the electrical resistivities ρ₁ and ρ₃ respectively of the first        elastomeric compound M₁ and of the third elastomeric compound M₃        are at most equal to 10⁶ ′Ω·cm, so that the bead layer, the        elastomeric coating compound of the carcass layer, the coating        compound of at least one crown layer, the base layer, and the        tread wing constitute a preferential conductive pathway of the        electric charges between the rim and the ground when the tire is        mounted on its rim and flattened on the ground, and the fourth        elastomeric compound M₄ of each sidewall has a viscous dynamic        shear modulus G″₄ at most equal to 0.125 MPa.

The essential idea of the invention is to simultaneously optimize thedesign of the sidewalls of the tire and that of its tread which isdivided into three portions: a central portion, and two tread wingslocated axially on either side of the central portion. These threeportions rest on a base layer radially on the inside of the tread. Theinvention relates both to the geometry and the physical properties ofthe elastomeric compounds of the tread, of the base layer and of thesidewalls of the tire.

According to the invention, the first elastomeric compound M₁ of atleast one tread wing has a thermal conductivity λ₁ at least equal to0.190 W/m·K.

Specifically, the elastomeric compound M₁ of the tread wing is incontact with the ground and, consequently, must be compatible with thegrip and wear performance requirements, in addition to the expectedelectrical properties. The tread wings thus have a sufficient thicknessto be in contact with the ground throughout the service life of thetire. Hence, in order to avoid the risks of bakelization of thiscompound, its thermal conductivity must be at a sufficient level toguarantee the conduction of the accumulated heat toward the outside ofthe tire. The inventors propose a level of at least equal to 0.190W/m·K. As regards the elastic shear modulus for generating thrust forcesboth in the circumferential direction and in the radial direction, alevel of 1.4 MPa is necessary.

Also according to the invention, the second elastomeric compound M₂ ofthe central tread portion has a viscoelastic loss tgδ₂ at most equal to0.06.

In a construction plant tire, the tread represents around 40% of thetotal volume of rubber and is, in fact, the main source of hysteresis.To improve the endurance, one of the solutions consists in obtainingelastomeric compounds of very low hysteresis in order to limit thetemperature level. By being free of the electrical resistivityconstraint for this elastomeric compound of the tread, in particular inthe central portion thereof, the composition may focus on the reductionof the hysteresis. Thus, a viscoelastic dynamic loss characterized by tg(δ_(max)) of the order of 0.06, measured at 100° C. and for a stressfrequency of 10 Hz, is obtained. The elastomeric compound of the centraltread portion consequently has a low hysteresis while having compatibleproperties for the wear and grip performance.

Again according to the invention, the third elastomeric compound M₃ ofthe base layer (33) has a thermal conductivity λ3 less equal to 0.190W/m·K.

For the base layer of the tread, the inventors focused on theformulation of an electrically-conductive elastomeric compound. Thiselectrical property was obtained by an addition of carbon black fillersin a proportion of 35 phr, which resulted in an electrical resistivityvalue of 10^(5.7) ′Ω·cm. This is a compromise between the electricalresistivity, the hysteresis level with a dynamic loss of 0.1, and thethermal conductivity with a value of 0.190 W/m·K. The base layer of thetread is a link in the pathway for discharging electrostatic chargeswhich is positioned between the tread and the compounds of the crownreinforcement. Its thermal conductivity level contributes to thedischarging of the heat from the inside of the tire outward.

Still according to the invention, the electrical resistivities ρ₁ and ρ₃respectively of the first elastomeric compound M₁ and of the thirdelastomeric compound M₃ of the base layer of the tread are at most equalto 10⁶′Ω·cm, so that the bead layer, the elastomeric coating compound ofthe carcass layer, the coating compound of at least one crown layer, thebase layer, and the tread wing constitute a preferential conductivepathway of the electric charges between the rim and ground when the tireis mounted on its rim and flattened on the ground.

Advantageously, the axial width (L₃₃₁, L₃₃₂) of the lateral portion ofthe base layer is at least equal to 200 mm.

Again advantageously, the base layer is formed by two separate lateralportions each having an axial width L₃₃₁ and L₃₃₂.

The base layer is formed by two separate lateral portions, therespective axial widths of which L₃₃₁ and L₃₃₂ are equal.

The base layer is formed by two separate lateral portions, respectivelyformed by the same third elastomeric compound M₃.

The objective of obtaining an electrically conductive tire results fromthe correct operation of the pathway for discharging the electrostaticcharges. The interfaces of the various constituents of the pathway fordischarging the electrostatic charges must be in contact, in twos, overa length of at least 10 mm, so as to always guarantee the continuity ofthe pathway for discharging the electrostatic charges to take intoaccount the manufacturing tolerances.

According to the inventors, the main role of the base layer is to ensurethe conduction of the electrostatic charges from the tread to thecarcass of the tire. Then the contact of the bead with the rim closesthe electrical circuit. Thus, there are several possible geometries forthe definition of the base layer. The final option will have to take ofits volume, its hysteresis, its thermal conductivity, and its industrialproduction cost during manufacture.

The base layer is formed by a single portion, in continuous contact withthe entire central tread portion and in contact at least partly with atleast one tread wing.

Unlike the prior art, the inventors here favour the electricalproperties of the base layer at the expense of the hysteresis.Specifically, even if the base layer extends over the entire width ofthe crown, its volume is limited by its thickness which is adverselyaffected at the industrially acceptable minimum value. For example, theinventors propose the thickness of from 7 mm to 18 mm. The optimizationof the hysteresis, in this case, is obtained by the formulation of thecompound of the central portion of the tread which represents thelargest volume of elastomeric compound, and therefore the greatestsource of hysteresis

According to the invention, at least one lateral portion of the baselayer is in contact at least partly with a tread wing over a length(L_(C1), L_(C2)) at least equal to 10 mm.

In one optimal embodiment of the invention where the exactly neededamount of material is used, the base layer is positioned so as to be incontact with the tread wing only over a length of 10 mm knowing that itswidth in the axial direction is at most 200 mm. It should be noted thatthe pathway for discharging the electrostatic charges is sufficient ifit is present only on a single side of the tire.

The first elastomeric compound M₁ of at least one tread wing (40) is anelectrically-conductive rubber composition based at least onpolyisoprene, on a crosslinking system and on at least one reinforcingfiller comprising carbon black, characterized by a BET surface area atleast equal to 110 m²/g and by a content at least equal to 30 phr and atmost equal to 80 phr.

The tread wings consist of an elastomeric compound intended to be incontact with the ground. In addition to the anticipated electricalproperties, the composition of the elastomeric compound should becompatible with the grip and wear performance requirements of the tire.The tread wings thus have a sufficient thickness to be in contact withthe ground throughout the service life of the tire. The reinforcingfillers of this elastomeric compound are in a sufficient amount, with acarbon black content of from 30 to 80 phr, and of appropriate quality,with a BET surface area of greater than 110 m²/g, to guarantee theelectrical conductivity of this elastomeric compound. As is known, theBET specific surface area of carbon blacks is measured according to thestandard D6556-10 [multipoint method (at least 5 points)—gas:nitrogen—P/P0 relative pressure range: 0.1 to 0.3]. The thermalconductivity is simultaneously adjusted to a level sufficient to ensurethe transfer of heat by conduction to the running surface of the tire.For example, a thermal conductivity value at least equal to 0.190 W/m·Kis suitable. The thermal transfer of the heat of the tread is alsocarried out by convection at the outer surface of the tire which is notin contact with the ground.

Preferentially, the two tread wings are formed by such an elastomericcompound, but, if a single tread wing is formed by such an elastomericcompound, the desired technical effect is also present. In other words,the solution proposed by the invention still remains valid for tireswhich would have a tread that is nonsymmetrical relative to theequatorial plane, with tread wings consisting of different elastomericcompounds. The presence of the pathway for discharging the electrostaticcharges on a single side of the tire is in principle sufficient.

According to a first embodiment of the central tread portion, formed bya second elastomeric compound M₂, the second elastomeric compound M₂ ofthe central tread portion is a rubber composition based on at least onediene elastomer, on a crosslinking system, and on a reinforcing fillercomprising carbon black, characterized by a BET surface area at mostequal to 115 m²/g and by a content at most equal to 40 phr, and silica,at a content at most equal to 15 phr. Advantageously, the mixture of theelastomer and carbon black is obtained beforehand via a liquid route.

In a construction plant tire, the tread represents around 40% of thetotal volume of rubber and is, in fact, the main source of hysteresis.To improve the endurance, one of the solutions consists in obtainingelastomeric compounds of very low hysteresis in order to limit thetemperature level. By being free of the electrical resistivityconstraint for this elastomeric compound of the tread, in particular inthe central portion thereof, the composition may focus on the reductionof the hysteresis, using, for example reinforcing fillers made of carbonblack and of silica in an elastomer obtained via a liquid route. To dothis, use is made of an elastomer in latex form in the form of elastomerparticles dispersed in water, and of an aqueous dispersion of thefiller, i.e. a filler dispersed in water, commonly referred to as“slurry”. Thus, a viscoelastic dynamic loss characterized by tg(δ_(max)) of the order of 0.06, measured at 100° C. and for a stressfrequency of 10 Hz, is obtained. The elastomeric compound of the centraltread portion consequently has a low hysteresis while having compatibleproperties for the wear and grip performance.

According to a second embodiment of the central tread portion, formed bya second elastomeric compound M₂, the second elastomeric compound M₂ ofthe central tread portion is a rubber composition based on at least onediene elastomer, on a crosslinking system, and on a reinforcing filler,at an overall content at most equal to 40 phr, and comprising carbonblack, and silica.

This alternative composition of the elastomeric compound of the centraltread portion meets the same requirement of minimizing the hysteresiswhile retaining properties in order to guarantee the grip and wearperformance.

According to a third embodiment of the central tread portion, formed bya second elastomeric compound M₂, the second elastomeric compound M₂ ofthe central tread portion is an electrically-conductive rubbercomposition based on at least one diene elastomer, on a crosslinkingsystem, and on a reinforcing filler comprising carbon black,characterized by a BET surface area at least equal to 120 m²/g and by acontent at least equal to 35 phr and at most equal to 80 phr, andsilica, at a content at most equal to 15 phr.

The presence of a pathway for discharging the electrostatic charges aspresented by the invention remains compatible with the use, in thecentral portion of the tread, of an electrically-conductive elastomericcompound. The compounds mainly filled with carbon black in amounts offrom 30 to 80 phr, and with a BET surface area of greater than or equalto 120 m²/g fall under this category.

According to the invention, the third elastomeric compound M₃ of thebase layer of the tread is an electrically-conductive rubber compositionbased at least on polyisoprene, on a crosslinking system and on at leastone reinforcing filler comprising carbon black, characterized by a BETsurface area at least equal to 110 m²/g and by a content at least equalto 30 phr and at most equal to 80 phr.

The base layer of the tread must above all be electrically conductive.It is a link of the pathway for discharging electrostatic charges whichconnects the tread in contact with the ground with the crown of thetire. Here in a cost optimization approach, the inventors have used thesame composition of compound as for the also electrically-conductivetread wing. The base layer of the tread represents a relatively smallvolume of rubber, thus emphasis is placed in the formulation on itsability to conduct the electrostatic charges. This objective is achievedwith the proportions of carbon reinforcing fillers of from 30 to 80 phr,with a BET surface area of 110 m²/g.

According to one preferred embodiment, the elastomeric compound M₄ ofeach sidewall layer has a rubber composition based on at least one blendof polyisoprene, natural rubber or synthetic polyisoprene, andpolybutadiene, on a crosslinking system, and on a reinforcing filler, atan overall content at most equal to 45 phr, and comprising carbon black,at a content at most equal to 5 phr, and, predominantly, silica, at acontent at least equal to 20 phr and at most equal to 40 phr.

On this axially outer portion of the sidewall, the composition of theelastomeric compound should lead to a reduction in the hysteresis.However, this drop in the hysteresis should be able to be achievedwithout deteriorating, in particular, the mechanical properties such asthe fatigue strength and, more particularly, the crack resistance.Indeed, the sidewalls of a construction plant tire are subjected to veryhigh stresses simultaneously in terms of bending strain, attacks andthermal stresses. These prolonged static or dynamic stresses of thesidewalls, in the presence of ozone, cause more or less pronouncedcrazing or cracks to appear, the propagation of which under the effectof the stresses may give rise to significant damage of the sidewall inquestion. It is therefore important for the elastomeric compoundsconstituting the tire sidewalls, for construction plant tires inparticular, to have very good mechanical properties, imparted inparticular by a high content of reinforcing fillers.

The architecture of the tire according to the invention will be betterunderstood with reference to FIG. 1, not to scale, which represents ameridian half section of a tire.

FIG. 1 schematically represents a tire 10 intended to be used on Dumpertype vehicles. FIGS. 2, 3, and 4 represent the various possibleconfigurations of the tread wings, of the base layer relative to thecentral portion.

In FIG. 1, the tire 10 comprises a radial carcass reinforcement 50,anchored in two beads 70 and turned up, in each bead, around a bead wire60. Each bead 70 comprises a bead layer 71 intended to come into contactwith a rim flange. The carcass reinforcement 50 is generally formed of asingle carcass layer, consisting of metal cords coated in an elastomericcoating compound. Positioned radially on the outside of the carcassreinforcement 50 is a crown reinforcement (not referenced), itselfradially on the inside of a tread 30. The tread 30 comprises, at eachaxial end, an axial end portion or tread wing 311 and 312, axially onthe outside of a central tread portion 32. The tread 32, and the treadwings 311, 312 rest on a radially inner base layer 311, 312. Each treadaxial end portion 311, 312 is connected to a bead 70 via a sidewall 20.

The radially outer top end of the tread wing 31 is in contact with thecentral tread portion 32 over its entire thickness. Its radially innerbottom end is in contact with the radially inner base layer of the treadover a length (L_(c1), L_(c2)) at least equal to 10 mm.

The base layer (33) is formed by two separate lateral portions (331,332) each having an axial width L₃₃₁ and L₃₃₂. In the example used thiswidth is equal to 200 mm.

The objective is to ensure a permanent contact between theelectrically-conductive elastomeric compounds, in twos, in order toguarantee the continuity of the pathway for discharging theelectrostatic charges, taking into account the manufacturing tolerances.

FIG. 2 represents a tread that is symmetrical relative to the equatorialplane comprising two axial end portions or tread wings that are actuallyseparated by a central portion. The inner end of the tread wing, in theaxial direction, is located at a given distance L₁ relative to theequatorial plane. The other outer end of the tread wing, still in theaxial direction, is positioned at a distance of L₂ of the sameequatorial plane. For the base layer, the axially inner (respectivelyaxially outer) end is located at a distance L₃ (respectively L₄) fromthe equatorial plane.

FIG. 3 represents a tread laid in a continuous base that spreads overthe entire width of the crown of the tire.

FIGS. 4 and 5 represent a tread that is not symmetrical relative to theequatorial plane. In FIG. 4, the tread wing is positioned only on thevehicle exterior side (reference 100), and in FIG. 5, it is positionedonly on the vehicle interior side (reference 110).

The invention has more particularly been studied on a tire for a Dumpertype vehicle, of dimensions 59/80 R63, in accordance with the invention,and as represented in FIG. 1.

The results calculated on the tire produced according to the inventionare compared to those obtained for a reference tire of the samedimensions, comprising a sidewall consisting of a single layer, and atread made of a single portion, in accordance with the prior art.

The inventors have established the connection between the chemicalcomposition of the elastomeric compounds and the physical parameterssuch as the electrical resistivity, the thermal conductivity, and theviscoelastic loss. By way of example, represented on the graph from theappended FIG. 6, for two elastomeric compounds, are the curves ofthermal conductivities as a function of the amount of reinforcingfillers in phr. These curves show that the elastomeric compound filledwith silica is optimized for the hysteresis, but with a thermalconductivity that is relatively lower than the elastomeric compoundfilled with carbon black, for which the electrical conductivity propertyis favoured.

According to the curve from FIG. 6, for a given content of filler, forexample carbon black, it is possible to predict the value of the thermalconductivity of the elastomeric compound. The thermal conductivities aremeasured at an ambient temperature of from 23° C. to 25° C. Thedependency of the thermal conductivity relative to the temperature isnot taken into account here.

The inventors determined the composition of the elastomeric compounds,constituting the sidewall layer, the tread wings, the central portion ofthe tread and the base layer by finding a compromise between thefollowing physical parameters:

-   -   the dynamic viscoelastic loss or the viscous shear modulus which        are directly connected to the viscoelastic heat sources;    -   the thermal conductivity which controls the thermal conduction        of the heat in the compounds;    -   the electrical conductivity which must be at a level sufficient        for discharging electrostatic charges.

In the example studied, the compositions of the elastomeric compounds,resulting from this compromise, are summarized in Table 1 below:

TABLE 1 Elastomeric Elastomeric Elastomeric compound M₁ compound M₂Elastomeric compound M₄ of the tread of the central compound M₃ of thesidewall Composition wing tread portion of the base layer layerElastomer NR 100 100 *  100 50 (Natural Rubber) Elastomer BR NC NC NC 50(Butadiene Rubber) Carbon black N330 NC NC NC NC Carbon black N234 35 35*  35 3 Silica (2) 10 10   10 29 Plasticizer (3) NC NC NC 10 Wax NC NCNC 1 Antioxidant 3 3   3 3 ZnO 2.7 2.7 2.7 2.5 Stearic acid 2.5 2.5 2.51 Sulfur 1.25  1.25 1.25 1 Accelerator 1.4 1.4 1.4 0.8 * elastomericcompound M₂ obtained via a liquid route (2) “Zeosil 1165MP” silica soldby Rhodia (3) “Vivatec 500” TDAE oil from Klaus Dahleke

Table 2 brings together the physical parameters of the elastomericcompounds, measured on test specimens and resulting from choices ofchemical composition:

TABLE 2 Elastomeric Elastomeric Elastomeric compound M₁ compound M₂Elastomeric compound M₄ of the tread of the central compound M₃ of thesidewall Composition wing tread portion of the base layer layer Thermal 0.240  0.240  0.240 0.208 conductivity at 25° C. (W/m · K) Electrical5.7  10.4  5.7  11.6   resistivity in Log (′Ω · cm) Viscous shear NC NCNC 0.125 modulus G″ max at 60° C. and 10 Hz (in MPa) Elastic shear 1.331.16 1.33 NC modulus G*max (50%, 100° C. and 10 Hz) Dynamic loss 0.100.06 0.10 NC tgδ_(max) (50%, 100° C. and 10 Hz)

In a construction plant tire, the amount of elastomeric compound of thetread represents around 35% to 40% of the total mass of elastomericcompounds of the tire. The tread is thus one of the main sources ofhysteresis, and it therefore contributes greatly to the increase intemperature of the tire. Consequently, the elastomeric compound M₂ ofthe central tread portion is designed to have a low hysteresis with adynamic viscoelastic loss of the order of 0.06, measured at atemperature of 100° C., and at a frequency of 10 Hz.

In one preferred embodiment of the invention, the elastomeric compoundM₂ of the central tread portion has a composition which comprises atleast one diene elastomer and a reinforcing filler that may be a blendof carbon black and silica, so that the carbon black has a content atmost equal to 40 phr and a BET surface area at most equal to 115 m²/gand the silica has a content at most equal to 15 phr. Theelastomer—carbon black mixture is obtained beforehand preferentially viaa liquid route.

In this embodiment, the central tread portion is electricallyinsulating. The discharging of the electrostatic charges is then carriedout along the conduction pathway defined by the invention which passesthrough the tread wings in contact with the ground and which are alwayselectrically conductive.

For the elastomeric compound M₁ of the running tread wings, the overallfiller content being 45 phr, with 35 phr of carbon black and 10 phr ofsilica, guarantees an electrical resistivity of less than or equal to10⁶ ′Ω·cm, and a suitable thermal conductivity. In the example dealtwith here, the thermal conductivity of the tread wing is equal to 0.240W/m·K. The same elastomeric compound M₁ is used for the two tread wingspositioned at the two ends of the tread, but the invention still remainsvalid if different materials are used. The required condition is to haveat least, at one of the two axial ends of the tread, an elastomericcompound with an electrical resistivity of less than or equal to 10⁶′Ω·cm.

In a tire for a construction plant vehicle, the mass of the elastomericcompounds of the sidewall is of the order of 15% of the total mass ofcompounds of the tire. The option selected by the inventors is to havean elastomeric compound for the sidewall of low hysteresis with aviscous shear modulus fixed at 0.125 MPa. Since the sidewall is notinvolved in the pathway for conducting electrostatic charges, there istherefore no need to fill the compound with for example carbon black. Afiller mainly with silica in a proportion of 29 phr, vs 3 phr carbonblack is used to achieve the target of low hysteresis.

The results on tires were obtained by finite element calculations is inorder to determine the viscoelastic heat sources, the temperature andthe electrical resistivity.

Finite element calculations were carried out on the tires of theinvention and reference tires respectively. The reference tire comprisesa standard sidewall layer and a standard tread. For example, thestandard sidewall layer is an elastomeric compound formulation accordingto the following proportions:

TABLE 3 Standard compound of the sidewall layer Composition of thereference tire NR (Natural Rubber) 50 BR (Butadiene Rubber) 50 Carbonblack N330 55 Carbon black N234 NC Silica (2) NC Plasticizer (3) 18 Wax1 Antioxidant 3 ZnO 2.5 Stearic acid 1 Sulfur 0.9 Accelerator 0.6

The standard tread of the reference tire comprises neither tread wingsnor base layer. It is made of a single portion.

The results of calculations for the reference tire are represented belowin Table 4:

TABLE 4 Tread made of a Results Single sidewall layer single portionElectrical 4.4 5.7 resistivity Log (′Ω · cm) Viscoelastic 4520 5100sources (W) Maximum 99.8 90 temperature° C.

The reference tire is electrically conductive with an average operatingtemperature of the order of 90.4° C.

For the tire of the invention, the results of the finite elementcalculations are summarized in Table 5:

TABLE 5 Single sidewall Base Tread Central tread Results layer layerwing portion Electrical 11.6 5.7 5.7 10.4 resistivity Log (′Ω · cm)Viscoelastic 2270 514 751 4540 sources (W) Maximum 88.9 78.5 65.5 85.9temperature° C.

The finite element calculations confirm the electrically insulatingnature of the sidewall layer and of the central tread portion. The treadwing in contact with the ground and the underlayer are, on the otherhand, electrically conductive. The evaluation of the electric potentialconfirms the conduction pathway with levels of electrical resistivityranging from 10⁴ ′Ω·cm to 10⁶ ′Ω·cm for the elastomeric compoundsconstituting the pathway for discharging the electrostatic charges.

For the tire of the invention, relative to the reference tire, theviscoelastic loss sources were halved in the sidewall of the tire, andin the tread the reduction is also significant.

As a consequence of the drop in the viscoelastic loss sources, thecalculation of the temperature field of the tire of the invention showsa maximum level of 88.9° C., which corresponds to a difference of 10%relative to the reference tire. This difference is sufficient for asignificant improvement in the endurance of the tire of the invention byprolonging its service life by around 30%.

The invention has been presented for a tire for a construction plantvehicle, but it can in fact be extrapolated to other types of tire.

1. A tire for heavy vehicle of construction plant type comprising: atread comprising two axial end portions or tread wings axially separatedby a central portion; a base layer, radially on the inside of the tread,comprising at least one lateral portion at least partly in contact withthe tread wing; a crown reinforcement, radially on the inside of thebase layer, comprising at least one crown layer, having metallicreinforcers that are coated in an electrically-conductive elastomericcompound; two sidewalls connecting the tread wings to two beads (70),adapted to come into contact with a mounting rim by means of a beadlayer made of electrically-conductive elastomeric compound; each saidsidewall being axially on the outside of a carcass reinforcementcomprising at least one carcass layer having metallic reinforcers thatare coated in an electrically-conductive elastomeric coating compound;at least one said tread wing having a first elastomeric compound M₁having a thermal conductivity λ₁ and an electrical resistivity ρ₁; thecentral tread portion having a second elastomeric compound M₂ having aviscoelastic loss tgδ₂; the base layer having a third elastomericcompound M₃ having a thermal conductivity λ₃ and an electricalresistivity ρ₃; each said sidewall having a fourth elastomeric compoundM₄ having a viscous dynamic shear modulus G″₄, wherein the firstelastomeric compound M₁ of at least one said tread wing has a thermalconductivity λ₁ at least equal to 0.190 W/m·K, wherein the secondelastomeric compound M₂ of the central tread portion has a viscoelasticloss tgδ₂ at most equal to 0.06, wherein the third elastomeric compoundM₃ of the base layer has a thermal conductivity λ₃ less equal to 0.190W/m·K, wherein the electrical resistivities ρ₁ and ρ₃ respectively ofthe first elastomeric compound M₁ and of the third elastomeric compoundM₃ are at most equal to 10⁶ ′Ω·cm, so that the bead layer, theelastomeric coating compound of the carcass layer, the coating compoundof the at least one crown layer, the base layer, and the tread wingconstitute a preferential conductive pathway of the electric chargesbetween the rim and the ground when the tire is mounted on its rim andflattened on the ground, and wherein the fourth elastomeric compound M₄of each said sidewall has a viscous dynamic shear modulus G″₄ at mostequal to 0.125 MPa.
 2. The tire according to claim 1, at least one saidlateral portion of the base layer having an axial width, wherein theaxial width of the lateral portion of the base layer is at least equalto 200 mm.
 3. The tire according to claim 1, wherein the base layer isformed by two separate said lateral portions each having an axial widthL₃₃₁ and L₃₃₂.
 4. The tire according to claim 1, wherein the base layeris formed by two separate said lateral portions, the respective axialwidths of which L₃₃₁ and L₃₃₂ are equal.
 5. The tire according to claim3, wherein the base layer is formed by two separate said lateralportions, respectively formed by the same third elastomeric compound M₃.6. The tire according to claim 1, wherein the base layer is formed by asingle portion, in continuous contact with the entire central treadportion and in contact at least partly with at least one said treadwing.
 7. The tire according to claim 1, wherein at least one saidlateral portion of the base layer is in contact at least partly with asaid tread wing over a length at least equal to 10 mm.
 8. The tireaccording to claim 1, wherein the first elastomeric compound M₁ of atleast one said tread wing is an electrically-conductive rubbercomposition based at least on polyisoprene, on a crosslinking system andon at least one reinforcing filler comprising carbon black, having a BETsurface area at least equal to 110 m²/g, and a content at least equal to30 phr and at most equal to 80 phr.
 9. The tire according to claim 1,wherein the second elastomeric compound of the central tread portion isa rubber composition based on at least one diene elastomer, on acrosslinking system, and on a reinforcing filler comprising carbonblack, having a BET surface area at most equal to 115 m²/g, and acontent at most equal to 40 phr, and silica, at a content at most equalto 15 phr.
 10. The tire according to claim 1, wherein the secondelastomeric compound of the central tread portion is a rubbercomposition based on at least one diene elastomer, on a crosslinkingsystem, and on a reinforcing filler, at an overall content at most equalto 40 phr, and comprising carbon black, and silica.
 11. The tireaccording to claim 1, wherein the second elastomeric compound M₂ of thecentral tread portion is an electrically-conductive rubber compositionbased on at least one diene elastomer, on a crosslinking system, and ona reinforcing filler comprising carbon black, having a BET surface areaat least equal to 120 m²/g, and a content at least equal to 35 phr andat most equal to 80 phr, and silica, at a content at most equal to 15phr.
 12. The tire according to claim 1, wherein the third elastomericcompound M₃ of the base layer of the tread is an electrically-conductiverubber composition based at least on polystirene, on a crosslinkingsystem, and on at least one reinforcing filler comprising carbon black,having a BET surface area at least equal to 110 m²/g, and a content atleast equal to 30 phr and at most equal to 80 phr.
 13. The tireaccording to claim 1, wherein the elastomeric compound of each sidewallhas a rubber composition based on at least one blend of polyisoprene,natural rubber or synthetic polyisoprene, and polybutadiene, on acrosslinking system, and on a reinforcing filler, at an overall contentat most equal to 45 phr, and comprising carbon black, at a content atmost equal to 5 phr, and, predominantly, silica, at a content at leastequal to 20 phr and at most equal to 40 phr.